The efficacy of radiation and chemical methods in the treatment of cancers has been limited by a lack of selective targeting of tumor cells by the therapeutic agent. In an effort to spare normal tissue, current tumor treatment methods have therefore restricted radiation and/or chemical treatment doses to levels that are well below optimal or clinically adequate. Thus, designing compounds that are capable, either alone or as part of a therapeutic method, of selectively targeting and destroying tumor cells, is a field of intense study.
Because of the known affinity of porphyrins to neoplastic tissues, there has been intense interest in using porphyrins as delivery agents in the treatment of neoplasms in the brain, head and neck, and related tumors. Porphyrins in general belong to a class of colored, aromatic tetrapyrrole compounds, some of which are found naturally in plants and animals, e.g., chlorophyll and heme, respectively.
Porphyrins and other tetrapyrroles with relatively long singlet lifetimes have already been used to treat malignant tumors using photodynamic therapy (PDT). In PDT, the patient is first injected with a photosensitizing drug, typically a porphyrin. The tumor cells, now photosensitized, are susceptible to destruction when exposed to an intense beam of laser red light. The biochemical mechanism of cell damage in PDT is believed to be mediated largely by singlet oxygen, which is produced by transfer of energy from the light-excited porphyrin molecule to an oxygen molecule. However, PDT has been limited predominantly by the photosensitizing compounds, which have lower than adequate selectivity to tumor cells and higher than optimal toxicity to normal tissue.
A promising new form of cancer therapy is boron neutron-capture therapy (BNCT). BNCT is a bimodal cancer treatment based on the selective accumulation of a stable nuclide of boron known as boron-10, or 10B, in the tumor, followed by irradiation of the tumor with thermalized neutrons. The thermalized neutrons impinge on the boron-10, causing a nuclear fission (decay reaction). The nuclear fission reaction causes the highly localized release of vast amounts of energy in the form of high linear-energy-transfer (LET) radiation, which can kill cells more efficiently (higher relative biological effect) than low LET radiation, such as x-rays.
In BNCT, the boron-containing compound must be non-toxic or of low toxicity when administered in therapeutically effective amounts, as well as being capable of selectively accumulating in cancerous tissue. For example, clinical BNCT for malignant brain tumors was carried out at the Brookhaven National Laboratory Medical Department using p-boronophenylalanine (BPA) as the boron carrier (Chanana et al., Neuro surgery, 44, 1182–1192, 1999). Although BPA has the advantage of low chemical toxicity, it accumulates in critical normal tissues at levels that are less than desirable. In particular, the tumor-to-normal brain and tumor-to-blood boron concentrations are in the ratio of approximately 3:1. Such low specificity limits the maximum dose of BPA to a tumor since the allowable dose to normal tissue will be the limiting factor.
A particular class of synthetic porphyrins, known as tetraphenyl porphyrins, have garnered intense interest in the design of new boron carrier compounds for BNCT. Tetraphenylporphyrins (TPPs) contain four phenyl groups, typically on the 5, 10, 15, and 20 positions of the porphyrin ring. An advantage of TPPs is their ease of synthesis.
The solubility of TPPs can be controlled by the substituents, generally on the phenyl positions. Those TPPs containing sulfonates or carboxylates are water-soluble. However, some of the carborane-containing TPPs have high lipophilic properties, which can require high amounts of non-aqueous excipients before administration into animals. High amounts of excipients may reduce the biological effect of the porphyrin by, for example, changing the microlocalization within the tumor cell such that it may be bound to membranes instead of being homogeneously distributed throughout the cell. In addition, the use of more hydrophilic bonds such as amide, ester, or urea bonds, although significantly more hydrophilic than carbon-carbon linkages, are known to hydrolyze under numerous types of conditions. Such hydrolysis is particularly problematic when such hydrophilic bonds are employed to attach the carboranyl group to the porphyrin molecule, since hydrolysis results in loss of the carbonyl group before reaching the target.
Therefore, there continues to be an effort to reduce the lipophilic behavior of TPPs while not compromising their chemical stability. For example, international Patent Application No. WO 01/85736 by Vicente et al describes the synthesis and use of tetraphenylporphyrin compounds that contain hydrophilic groups. A salient feature of the Vicente compounds is the attachment of the carboranyl group to the phenyl group by, exclusively, a carbon-carbon linkage. Although such a carbon-carbon linkage is not prone to hydrolysis or other chemical attack, such a linkage is significantly hydrophobic.
Porphyrins also have the advantage of having the ability to chelate metal ions in its interior. Such chelated porphyrins can additionally function as visualization tools for real-time monitoring of porphyrin concentration and/or diagnostic agents. For example, when chelated to paramagnetic metal ions, porphyrins may function as contrast agents in magnetic resonance imaging (MRI), and when chelated to radioactive metal ions, porphyrins may function as imaging agents for single photon emission computed tomography (SPECT) or positron emission tomography (PET).
In addition, by using chelated boron-containing porphyrins in BNCT, boron concentration and distribution in and around the tumor and all tissues within the irradiated treatment volume can be accurately and rapidly determined noninvasively before and during the irradiation. Such diagnostic information allows BNCT treatment to be performed more quickly, accurately, and safely, by lowering exposures of epithermal neutrons in regions of tissues known to contain high levels of boron. Short irradiations would obviate the inconvenience and discomfort to the patient of long and often awkward positioning of the head at a reactor port. However, the anticipated use of acceleratorgenerated neutrons would likely produce a significantly lower flux and hence effect longer irradiation times, so that compounds that have longer tumor retention times would become critical.
Accordingly, there is a need for new compounds, especially boron-containing porphyrins, with long retention times in tumors, and that selectively target and destroy tumor cells with minimal damage to normal tissue. In addition, there is a need for more effective methods for the treatment of brain, head and neck, and related tumors, and more particularly, more effective BNCT treatments and boron-delivery compounds used therein.